Generally, silver halide emulsions are prepared through such stages that a soluble silver salt is mixed with a soluble halide in an aqueous gelatin solution to form silver halide grains and the resulting emulsions are desalted and then chemically sensitized.
It is known that sensitizing dyes may be used as chemical sensitization aids by adding them in the chemical sensitization stage to control chemical sensitization nuclei, improve high illumination non-linearity and inhibit inherent desensitization. These methods are described in, for example, JP-A-58-113926 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-58-113927, JP-A-58-113928, U.S. Pat. Nos. 4,439,520 and 4,435,501, Research Disclosure, Item 17643, section III, JP-A-62-6251, JP-A-58-126526, JP-A-62-56949, JP-A-62-43644, JP-A-58-113928, Japanese Patent Application No. 62-203635, JP-A-1-40938, JP-A-1-62631, JP-A-1-62632, JP-A-1-74540, JP-A-1-158425 and JP-A-2-34.
Examples of the sensitizing dyes which can be added include cyanine dyes, merocyanine dyes, complex cyanine dyes, holopolar cyanine dyes, hemicyanine dyes, styryl dyes and hemioxanol dyes. For example, reference can be made to JP-A-61-160739.
When the sensitizing dyes are conventionally used as chemical sensitizing aids, sensitizing dyes having absorptions according to the purpose of spectral sensitization are always allowed to be previously adsorbed by the silver halide emulsion grains to control the formation of chemical sensitization nuclei before chemical sensitization is carried out.
In this case, a problem is often caused in that photographic performance is deteriorated by development restraint, residual color, inherent desensitization, fog, etc. due to adsorbed dyes. In addition, when the sensitizing dyes are adsorbed and spectral sensitization is then carried out, the sensitizing dyes used as the chemical sensitization aids often exhibit unfavorable spectral characteristics. Even when sensitizing dyes favorable to spectral sensitization are used as chemical sensitization aids, unfavorable chemical sensitization characteristics sometimes occur.
Accordingly, there are cases in which sensitizing dyes favorable to spectral sensitization are often not preferred as the chemical sensitizing aids, while sensitizing dyes preferred as the chemical sensitizing aids are often unfavorable for spectral sensitization.
Under such circumstances, attempts have been made to develop sensitizing dyes serving as excellent chemical sensitization aids as well as excellent spectral sensitizing agents. However, sensitizing dyes having both functions are rare.
Accordingly, it has been demanded to develop a method wherein the function of controlling chemical sensitization nuclei and the function of carrying out spectral sensitization are separately carried out in the technical field in which sensitizing dyes are used as chemical sensitization aids.
When sensitizing dyes represented by general formula (D-2) described hereinafter are used as the chemical sensitization aids, there is a side effect of increasing fog. Hence, it is demanded to decrease this undesirable side effect. As a method for solving this problem, a method has been proposed wherein nitrogen-containing heterocyclic compounds are used together with the above sensitizing dyes as described in JP-A-62-89952. However, the improvement is still insufficient.
Under such circumstances, it has been demanded to develop a method wherein the sensitivity of silver halide emulsions which are spectrally sensitized by sensitizing dyes represented by general formula (D-2) described hereinafter can be increased and fogging can be reduced.
Further, an increase in the fogging of the silver halide emulsions spectrally sensitized by general formula (D-2) has become a serious problem in the field of color diffusion transfer photographic materials because rapid processing has been demanded.